Pilot device for a suspended knife of a cutting machine for cutting sheet material

Abstract: Pilot device for a suspended cutting knife of a cutting head of an automatically controlled cutting machine for cutting fabric sheet material spread out on a cutting table in multiple layers, which cutting head being controlled according to a three-dimensional coordinate system by means for moving the cutting head along the X- and Y-axis and pivoting about the Z-axis for moving the knife tangently along a predetermined cutting path during cutting of said material, said cutting knife is mounted reciprocally movable along said Z-axis, said cutting head further comprises guiding means for guiding the unsuspended part of the cutting knife in the cutting head and a presserure foot rigidly connected to the pivotable cutting head. The suspended end of the knife (10) is mounted in the cutting head freely rotatable around the Z-axis, and the guiding means arranged adjacent to the free end of the knife comprising a socket (53) rigidly connected to the cutting head having a support (60) being freely rotatable mounted in said socket (53) around a vertical axis (6) located, in front and adjacent to the cutting edge (11) of said cutting knife and parallel in a predetermined distance to the Z-axis (5) of the coordinate system. Said support (60) having a slot (61) surrounding the flanks (12,13) of the knife (1), which slot (61) is excentrically positioned to said vertical axis (6) and extends to the trailing edge (12) of the knife.

Description: The invention relates to a cutting head of an automatically controlled cutting machine for cutting fabric sheet material spread out on a cutting table in multiple layers, the cutting head having a cutting knife suspended therefrom and being controlled in a three-dimensional coordinate system by means for moving the cutting head along the X- and Y- axis and rotating it about the Z-axis so that the knife can be moved tangently along a predetermined cutting path during cutting of said material, said cutting knife being mounted reciprocally movable along said Z-axis, said cutting head further comprising guiding means for guiding the cutting knife, relative to the cutting head, at a location adjacent to its free end, and a pressure foot rigidly connected to the cutting head. Automatically closed loop controlled cutting machines for cutting sheet material as fabrics for garments spread out on the cutting table in multiple layers being held onto the cutting table by atmospheric pressure are wellknown. One of the problems of such cutting machines is that without corrective measures the knife will track a cutting path in the upper ply of the layup slightly different from the cutting path in the lower ply so that the pattern pieces from the respective plies will have slightly different shapes. Therefore the height of the staple of layers to be cut is limited by the knife bending stiffnes for a desired cutting quality. Known means for compensating defects depending on bending flexure of the knife of an automatic cutting machine comprising sensors for sensing the lateral forces acting on the flanks of the knife during cutting. These signals are transferred and applied to a computer or processor which provides correcting signals representing correction angles to be superimposed on the orientation of the preprogrammed cutting path of the knife around the Z-axis with respect to its path; see US 4,133,235. According to GB-2 094 031 digital sensors are used for detection the bending of the knife and providing signals indicating the presence of flexure and its direction. By feed back knife position to a servomechanism the required correction is computed in conjunction with these signals. According to both known methods the required correction of the knife angle has to be computed in conjunction with lateral force signals and information concerning the properties of the material to be cut in order to minimize defects depending on knife flexure. Therefore such methods require that lateral forces acting on the knife are correctly measured and transformed into correcting signals to modify the preprogrammed orientation of the knife around its longitudinal or Z-axis and require further a relative great expenditure in sensors, transducers, actuator and in data logger feedback gauging system which are very complex and thus quite expensive and are further difficult to handle. According to experience, loads acting on the knife during cutting operation are of different types; one of these are lateral loads effecting knife bending. These lateral loads acting onto the flanks of the knife are caused by the pressure of the fabric to be cut during interaction of the cutting knife and sheet material, which generates friction loads in the feeding direction of the moving knife also. The pressure of the fabric to be cut can be different at both sides of the knife due to different reasons, as the anisotropy of the fabrics or the proximity of a previous cut or the fabric border at one side of the knife. The relations between lateral pressure and knife bending without evaluating other dependences are generally indicated in Figures 2 and 3. Under the adoption that the pressure on a point of the knife is proportional to the compression of the fabric in this point the following correlations are applicable. In Figure 2 which shows a section view on a staple of layers whereby line "t" is the theoretic path which is the path followed by the knife without bending whereas line "r" is the actual path in said section due to knife bending. Deformation in this section is "d", thus the pressure can be expressed by p = K · y "K" being a constant that, in general, can be different at each side of the knife due to the anisotropy or the proximity to a previous cut line, as said before. Figure 3 shows the assumption that p'= K' · y > p = K · y If the knife could pivot in relation to its path around an axis near its leading edge the pressure appearing at each flank of the knife will change according to the distances "y" of every point to the cut line as shown in Figure 3. The rotation about this axis in front of the knife would tend to balance said lateral loads, which leads to a decreasing or avoiding of the bending on the knife and the deformation "d". Therefore a general object of the present invention is to provide a new device for minimizing the defects depending on bending of a cutting knife of a cutting machine while being in cutting position along a predetermined cutting path without measuring lateral forces acting on the flanks of the knife and without demand for superimposing a correction angle to the predetermined orientation angle of the knife in relation to the cutting path. On the basis of the foregoing criteria the general object of the invention is accomplished by a cutting head as defined in claim 1. Further object of the invention is to specify a structure for said guiding means which is reliable in operation, easy to manufacture and in service. In the cutting head according to claim 1, this further object is accomplished by providing for the additional features of claim 2. Futhermore, preferably the support is located in the area of the pressure foot. An embodiment of the invention will now be described by way of example and with reference to the accompanying drawing in which Figure 1 shows an schematical perspective view of an automatic controlled cutting machine for cutting multiple layered sheet material hold by atmospheric pressure, Figure 2 shows an orthogonal section through the cutting knife and the unequal load distribution acting on the flanks of the knife during cutting, Figure 3 shows an orthogonal section through the cutting knife according Figure 2 with balanced load distribution acting on the flanks of the knife, Figure 4 shows an isometric view of a part of a cutting head of a cutting machine according Figure 1 having a pilot device according to the invention and Figure 5 shows the geometric relation of the knife of the cutting head according to the invention in different sectional orthogonal cuts due to the balance effect of the pilot device according Figure 4. Referring to Figure 1 the present invention relates to an automatically controlled cutting machine 100, in which a staple 102 of layers of fabric material to be cut is fed from a suitable supply means at one end of a cutting table and is passed over a cutting table 103. On the surface of said cutting table 103 these sheets of fabrics are spread for cutting by a cutting tool reciprocally movable mounted in a cutting head 104 which is mounted on an X-Y-carriage 105 for moving over the cutting table along X-Y-coordinates. The cutting tool is a suspended blade or knife and reciprocatable along its longitudinal axis pivotable mounted in the cutting head 104 which follows a predetermined cutting path by servomotors. The pivot axis of the cutting knife is the Z-axis which is perpendicular to the cutting surface of a three-dimensional coordinate system X, Y, Z of the controlling means for generating the cutting path of the knife. According to the predetermined controlled motion of the X-Y-carriage and the motion of the knife around Z-axis the cutting edge of the knife remains tangently to said cutting path represented by line "t" in Figure 2. These movements are controlled by a controller 107. The cutting table has evacuation means (not shown) in order to evacuate the cutting table 103 for holding said staple in a defined position by atmospheric pressure. The cutting surface of the cutting machine is penetrable by the cutting knife in wellknown manner. Neither the servo-motors for driving the X-Y-carriage in X- and Y-direction and the cutting head around Z-axis nor the motor and power transmission for the reciprocating movement of the cutting knife are shown. Figure 4 illustrates a part of the cutting head 104 comprising a shaft 50 housing a mount 51 which is reciprocally movable guided within the shaft 50 along and freely rotatable about an axis 5 which is the Z-axis of the coordinate system. On the mount 51 a knife 10 is suspended having a cutting or leading edge 11, a trailing edge 12 and two flanks 13 and 14 between leading and trailing edge. A pressure foot 52 is rigidly, but adjustably connected to the shaft 50 for lying on the upper layer of the staple of the fabric sheet material. A socket 53 is rigidly connected to the shaft 16 and adjacent to the pressure foot. In the socket 53 a support 60 is freely rotatable mounted about a vertical axis 6 which is in front and adjacent to the cutting edge 11 of the cutting knife 10 and parallel in a predetermined distance to the Z-axis (5) of the coordinate system, thus the cutting edge is placed between the Z-axis and the vertical axis 6. In the support 60 is a slot 61 excentrically arranged to said vertical axis 6 and surrounding the cutting knife near the free end of the knife. The inner surfaces of the slot 61 acting as a lateral operative glide bearing for the trailing edge 12 and the flanks 13 and 14 of the knife while the knife is in reciprocating movement; for example driven by a electromagnetic linearmotor. According to the arrangement of the just described knife 10 which moves up and down along the shaft 50, but is free to rotate independently within said shaft 50 around the vertical axis 5. As said before, the curve paths are followed by rotating shaft 50 by means of -servomotor and drive (not shown) in accordance to the controller commands. Lower support 60 can rotate around axis 6, which is placed in front of the leading edge 11 of the knife 10, at a determined distance of axis 5 (Z-axis). Both axis 5 and 6 provide to the knife 10 a determined position with respect to the assembly, as it can not freely rotate around both axis simultaneously. Thus, if the assembly rotates around axis 5 by command of the controller, the knife 10 will also rotate around this axis; this is the case to follow a predetermined curve path commanded by the controller 107 in well known manner. Under this normal condition, that means without loads acting onto the flanks of the knife 10, socket 53 and support 60 will turn around axis 5 - namely the Z-axis - simultaneously according to the preprogrammed cutting path as the knife 10 acts as a dog. During the cutting operations, different pressures at both flanks 13 and 14 of the knife 10 will appear then causing lateral loads and deformations as said before. Due to the knife support conditions the lateral loads that appear on the knife in the range of the cutting area are supported in the support 60 and cause a twisting of said support around the axis 6. This twisting is limited by the bending stiffness of the knife 10 between said support 60 and the upper end suspended to mount 51, see Fig. 5 which shows schematically a plan view of the different knife sections when laterally loaded. Section 70 is a plan view of the knife 10 before loading. Section 80 and 90 reffers to a laterally loaded condition of the knife. Section 80 is illustrating a knife section at the level of support 60 the knife has twisted under lateral loads together with support 60 around axis 6 which remains in its original position as axis 6 is stiffly joined to shaft 50. The section 90 is the suspended knife section at the upper end, connected to mount 51, the knife has twisted around the axis 5. The bending deformation "f" between sections 80 and 90 is related to the angle of twisting and both of them depend on the lateral loads and the knife bending stiffness. With other words: Due to the free pivotability of the mount 51 about the axis 5 with respect to the shaft 50 and of the support 60 about axis 6 with respect to the shaft 50 the knife 10 can twist in the region between the slot 61 and the mount 51 due to the loads on the flanks and due to the straining capability of the knife as the leading edge of the cutting knife 10 lags behind the advancing axis 6 seen in feeding direction. The knife torque stiffnes is of the same order than the knife bending stiffnes but the torque moment due to the pressure loads is much lesser than the bending moment and therefore the strain to torsion is considered neglectiable. That means that every knife section of the free end of the knife 10 will twist almost the same angle around the longitudinal axis of the knife until the forces acting on the flanks of the knife are equal. This self balancing effect is shown in Figure 3 and 5. When the cutting knife 10 is laterally unloaded any of the orthogonal sections along the knife are congruent as seen indicated by number 70 and the Z-axis (5) and axis 6, respectively intersect perpendicular the tangent to the predetermined cutting path; see Figure 5. While being in cutting condition along a curved cutting path laterally loads appears to the flanks of the cutting knife and thus to the inner surfaces of the slot 61. Under the influence of the lateral loads the knife will twist due to the free rotability of the support 60 with respect to the cutting head 104 whereby rotation of the cutting head 104 about Z-axis is representing the actual tangent angle to the predetermined cutting path as it is seen in Figure 5 indicated by number 80. This twisting effect is becoming an equilibrium state at that condition when the lateral loads are balanced by the torque stiffnes of the cutting knife - which is slidable guided between the inner surfaces of the slot 61 of the support 60 - and the interrelationship of the excentrically arranged vertical axis 6 of the support 60 in relation to the mount 51 guided by the shaft 50 rigidly associated to the cutting head 104. Under this circumstance the vertical axis 6 remains in the same position while the intersection point of axis 5 moves to 5' - section 80 of Figure 5 - , however axis 5 remains in the same position in the suspended section of the cutting knife as is referenced by number 90 in Figure 5. The cutting knife twists together with the suspension mount 51 around axis 5 in the same angle 7 as the support 60 twists around the axis 6 as shown in Figure 5. The displacement "f" and the angle 7 twisted under influence of the lateral loads onto the flanks of the knife are geometrically related by the position of axis 5 and 6. The displacement "f" is a function of the load and the knife bending stiffness. This function can be optimized for instance by using spring means acting on the support 60. Under these conditions the deviation "d" of the actual cutting path with respect to the predetermined theoretic cutting path will become a minimum. Thus the deviation of knife sections beneath the pressure foot 52 within the staple of layered sheet material would be minimized as well. From the foregoing description it can be seen that the knife bending effect during the cutting operation of the sheet material can be deminished close to zero without need of a measurement of forces acting on the flanks of said knife whereby the subsequent data processing is simple and in a small range.

Claims: Cutting head of an automatically controlled cutting machine for cutting fabric sheet material spread out on a cutting table in multiple layers, the cutting head (104) having a cutting knife (10) suspended therefrom and being controlled in a three-dimensional coordinate system by means for moving the cutting head (104) along the X- and Y- axis and rotating it about the Z-axis so that the knife (10) can be moved tangently along a predetermined cutting path during cutting of said material, said cutting knife (10) being mounted reciprocally movable along said Z-axis, said cutting head further comprising guiding means (53, 60) for guiding the cutting knife (10), relative to the cutting head (104), at a location adjacent to its free end, and a pressure foot (52) rigidly connected to the cutting head (104), characterized in that the end of the knife (10), by which it is suspended from the cutting head (104), is mounted in the cutting head guided for free rotation around the Z-axis, that the guiding means arranged adjacent to the free end of the knife (10) comprises a socket (53) rigidly connected to the cutting head and having a support (60) mounted therein, the support (60) being guided for free rotational movement in said socket (53) around a vertical axis (6) located in front and adjacent to the cutting edge (11) of said cutting knife, the vertical axis (6) being parallel to the Z-axis (5) of the coordinate system and located in a predetermined distance therefrom, said support (60) having a slot (61) surrounding the flanks (12, 13) of the knife (1), the slot (61) being eccentrically positioned relative to said vertical axis (6) and extending to the trailing edge (12) of the knife such that the knife (10) can twist about the vertical axis (6) under the influence of lateral loads generated during cutting between said sheet material and the flanks (13, 14) of the knife (10) into a balanced equilibrium state. Cutting head according to claim 1, characterized in that it comprises a shaft (50) by which it is rotatable about the Z-axis, said socket (53) being rigidly connected to the shaft (50) and said shaft (50) housing the cutting knife's reciprocally movable suspension mount (51). Cutting head according to claim 1 and 2, characterized in that the support (60) is located in the area of the pressure foot (52). Cutting head according claims 1 to 3, characterized by a spring acting on the support (60) to assist minimizing displacement (d) between predetermined and real cutting line (t, r).

Inventors: CRISTO JUAN CARLOS, GALAN MARIO ANDRADA, PEREZ BERNARDO ALCANTARA, CRISTO, JUAN CARLOS, GALAN, MARIO ANDRADA, PEREZ, BERNARDO ALCANTARA
Applicants: INVESTRONICA SA, INVESTRONICA S.A.
Application year: 1994
Main classification: B26D7
IPC codes: B26D7, A41H43, B26F1, B26D1